Homework Help:
Force Analysis

Hi all,
I am struggling on this force analysis questions. . Essentially my problem is to determine the directions of all the forces. Once I do that I can solve it using vector diagrams, or [tex]\sum{M} = 0[/tex] and [tex]\sum{F_x}=0[/tex] and [tex]\sum{F_y}=0[/tex], although it is clearly looks much more convieniant to use vector diagrams. ( I think it is to scale)

What is M in your equation? And what's your specific problem in determining the directions of the forces?

Draw a vector diagram (a freebody diagram) to show the forces and their relative inclinations, at each point in question. For the directions, note that you are given the perpendicular distance and the base distance for the triangles involving these inclination angles. Use for instance,

M was moment. My big problem is the direction of the forces? Is the force at a A directly down? Or is it along the line AB? What about the force at B? Intuituions tells me it will get pushed left. Right?

Okay I understand your problem now. Well have a look at your diagram as you read this:

There will be a force on B acting downwards along AB (pointing towards B) due to the link AB shown. The reaction force will act along BA (pointing towards A) on the link BA. There will be a second force on B acting along CB (pointing towrads B). The reaction force will act along BC (pointing towards C) on the link BC.

I somehow find the idea of force at a point dubious in this case unless you say that the force is acting at a point (the word acting does mean a lot as you can for that matter consider any point in space and translate the force vector there and say that so and so force is at a point P, whereas actually it could be acting not on P but on Q and you shifted it! This would change your moment equations too and you could get confused.). You must remember that every force has a reaction force associated with it which acts back on the body producing that force (Newton's Third Law).

So you need to do some spadework to figure out through a freebody diagram (which is an absolute MUST for such problems) what the directions of the forces acting at a point are first. Once you know the directions, you can choose a coordinate system with axes parallel and perpendicular to the baseline (horizontal) shown in the diagram and resolve your forces accordingly--using the other data given to you about the 100N net force at A.

Now with all this info, try and figure out what the force components along the coordinate system chosen at A, B, C, D and E are in terms of the angles of the triangles involved (and let us know if you have any further difficulties). Please do the problem algebraically first taking the angles as (theta)1, (theta)2 ... and the forces as F1, F2, etc (according to your convention/wish). Substitute the values in the last step so that if you go wrong on the way you can correct your algebra.